Laser-produced plasma (LPP) devices are an attractive source of X-rays or short-wavelength radiation due to their relative small size, high brightness and high spatial stability. Two established applications for LPP are microscopy and lithography. However, conventional LPP devices utilize solid targets that produce debris that may easily contaminate, coat, or destroy sensitive X-ray components, such as optics or zone plates, that are positioned close to the plasma. Unfortunately, increasing the distance or introducing filters in order to protect the components typically reduces the amount of radiation that can be captured or utilized.
For convenience, solid targets have often been used for LPP soft X-ray sources. Examples of solid target LPP systems are described in U.S. Pat. Nos. 5,539,764; 6,016,324; 6,307,913; and 6,707,101, all of which are hereby incorporated by reference herein in their entirely for all purposes. In general, targets formed from materials having low molecular weights yield emission spectra that are very narrow, while targets formed from materials having high molecular weights yield emission spectra having continuum radiation due to Brehmsstrahlung emission. Thus, low molecular weight targets are desirable for LPP applications. Unfortunately, with low molecular weight targets, significant amounts of debris, e.g., hot ions and larger particles, are created. In addition, such debris often follows the generated X-rays out of the laser ablation chamber of the LPP device, which can contaminate or damage components outside of the chamber as well.
Several methods have been developed to reduce the effect of debris, such as using a small back pressure of helium or other gas, or strategically locating a relay mirror for the protection of sensitive components. Additionally, thin film tape targets, which are becoming more commonplace, help reduce the amount of debris by avoiding shock wave ejection or delayed evaporation. Unfortunately, significant amounts of debris particles are produced, presumably from cooler zones illuminated by the noncentral parts of the laser beam. Gas-phase targets have been another low-debris alternative; however, such low density results in low X-ray intensity.
Other approaches to debris reduction have included waterjet devices and liquid droplets used for the laser target. However, even these approaches still result in the creation of some amount of debris when the X-rays are generated, and thus the potential for debris escaping the vacuum chamber and potentially contaminating and/or damaging outside components still exists. Accordingly, since the production of debris within a typical LPP device is difficult to eliminate, devices and methods are needed to prevent the LPP-generated debris from exiting the vacuum chamber and damaging delicate components of the LPP device.